1. Field of the Invention
The present invention relates to a heat pipe for transporting heat as latent heat of a working fluid or a condensable fluid, and relates especially to a vapor chamber in which a sealed receptacle is shaped into a tabular shape, i.e., a flat rectangular plate, and which is constructed to create a pumping force for refluxing a liquid phase working fluid to a portion where it evaporates, by means of a capillary pressure.
2. Discussion of the Related Art
In the customary way, a heat pipe for transporting heat in the form of latent heat of a working fluid is well known in the prior art. The heat pipe of this kind is a heat conducting element encapsulating a condensable fluid such as water in a sealed receptacle (container) after evacuating an air therefrom. Such a heat pipe is constructed to transport the heat as latent heat of a working fluid by evaporating the working fluid, with the heat input from outside, and by condensing a vapor by radiating the heat after the vapor flows to a condensing part of a low temperature and a low pressure. Accordingly, since the heat is transported in the form of latent heat of the working fluid, the heat pipe has more than ten times to several hundred times of heat transporting capacity in comparison with that of copper which is known to have the highest heat conductivity.
According to a heat pipe of this kind, the heat is transported by means of flowing the evaporated vapor phase working fluid to a condensing part in a low temperature and low pressure side, and, after the heat transportation, the condensed liquid phase working fluid is refluxed to the evaporating part (i.e., a heat inputting part) by the capillary pressure of a wick.
The wick is, in short, a member for creating a capillary pressure, and therefore, it is preferable that it be excellent in hydrophilicity with the working fluid, and it is preferable that its effective radius of a capillary tube as small as possible at a meniscus formed on a liquid surface of the liquid phase working fluid. Accordingly, a porous sintered compound or a bundle of extremely thin wires generally is employed as a wick. Among those wick members according to the prior art, the porous sintered compound may create great capillary pressure (i.e., a pumping force to the liquid phase working fluid) because the opening dimensions of its cavities are smaller than that of other wicks. Also, the porous sintered compound may be formed into a sheet shape so that it may be employed easily on a flat plate type heat pipe or the like, called a vapor chamber, which has been attracting attention in recent days. Accordingly, the porous sintered compound is a preferable wick material in light of those points of view.
The heat transporting characteristics of the heat pipe including the vapor chamber is thus improved as a result of an improvement of a wick material and so on, and miniaturization is also attempted in connection with this. At the same time, the cooling of a personal computer, a server, or a portable electronics device, which are enhanced in compactness and capacity, has been becoming a problem in recent days. The heat pipe has been garnering the attention as a means for solving this problem, and it has been employed more frequently. Examples of employing such downsized and thin-shaped heat pipe are disclosed in Japanese Patent Nos. 2,794,154 and 3,067,399, and Japanese Patent Laid-Open No. 2000-49266.
As described above, it is possible to increase the capillary pressure for refluxing the liquid phase working fluid if a porous body is employed as a wick to be built into the heat pipe. This is advantageous for downsizing the vapor chamber. However, a flow path is formed by the cavity created among the fine powders as the material of a porous body, so that the flow cross-sectional area of the flow path has to be small and as intricate as a maze. Therefore, it is possible to enhance the capillary pressure which functions as the pumping force for refluxing the liquid phase working fluid to a portion where it evaporates. However, on the other hand, there is a disadvantage because the flow resistance against the liquid phase working fluid is relatively high. For this reason, if the input amount of heat from outside increases suddenly and drastically, for example, the wick may dry out due to a shortage of the liquid phase working fluid to be fed to the portion where the evaporation of the working fluid takes place.